Transducer with integrated sensor

ABSTRACT

An electroacoustic transducer includes a housing and a moving portion adapted to move relative to the housing. The moving portion has a first surface and includes a first electrode. A second electrode on a first surface of the housing is located proximate to a first region of the first electrode, and a third electrode on the first surface of the housing is located proximate to a second region of the first electrode. A capacitance between the first and second electrodes and a capacitance between the first and third electrodes vary similarly with displacement of the moving portion relative to the housing. An impedance buffer is coupled to the second electrode and the third electrode.

BACKGROUND

This disclosure relates to an electromechanical transducer with anintegrated sensor for measuring displacement and its derivatives.

An electromechanical transducer with such an integrated sensor permitsfeedback control systems to directly measure displacement of the movingportion of an electromechanical transducer. Displacement measurementscan be derived to obtain velocity, acceleration, and jerk. One or moreof these measurements can be directly or indirectly used by a feedbackcontrol system for system control.

SUMMARY

In general, in some aspects, an electroacoustic transducer includes ahousing and a moving portion located proximate to the housing andadapted to move relative to the housing. The transducer also includes adisplacement sensor, which includes a first electrode adhered to themoving portion of the transducer, a second electrode on a first surfaceof the housing, located proximate to a first region of the firstelectrode, and a third electrode on the first surface of the housing,located proximate to a second region of the first electrode. A firstcapacitance between the first electrode and the second electrode and asecond capacitance between the first electrode and the third electrodeeach vary similarly with a displacement of the moving portion relativeto the housing. An impedance buffer is coupled to the second electrodeand the third electrode.

Implementations may include one or more of the following. The change indistance between a first surface of the housing and the first surface ofthe moving portion resulting from movement of the moving portion issubstantially uniform over the area of the second and third electrodes.The impedance buffer may include a bias voltage source providing a fixedcharge to at least one of the electrodes of the displacement sensor, andan amplifier amplifying a change in voltage between the first and secondelectrodes to produce an output voltage between first and second signaloutputs. The moving portion may be metal and may include the firstelectrode. The moving portion may include a diaphragm.

The first surface of the housing may be a surface of a printed circuitboard, and the second and third electrodes may be formed from metalareas on the printed circuit board. The housing may include a basket.The housing may include a pole piece. The moving portion may include avoice coil mechanically coupled to a diaphragm. The moving portion mayinclude a magnet mechanically coupled to a diaphragm. The layer of metalof the first electrode may include a coating on a non-conductivesubstrate.

The transducer may include a compression-type electroacoustictransducer, with the housing including a phase plug and the secondelectrode and third electrode formed from metal on a surface of thephase plug. The phase plug may include a plurality of distinct parts,and the second and third electrode may be formed from layers of metalconforming to portions of a surface of one of the plurality of parts ofthe phase plug, or they may be formed from solid metal portions of oneof the plurality of parts of the phase plug. The first surface of thehousing may be a surface of a block of conductive material, and thesecond and third electrodes may be formed from portions of the blockthat are electrically insulated from each other. The first surface ofthe housing may be a surface of a block of non-conductive material, andthe second and third electrodes may be formed from layers of metaladhered to the block of non-conductive material.

The amplifier may include a transistor having its gate coupled to afirst terminal of the displacement sensor and its source and draincoupled to a first signal output and a second signal output. The biasvoltage source may include an external power source having a firstterminal coupled to the drain of the transistor and a second terminalcoupled to a second terminal of the displacement sensor, the biasvoltage being applied to the at least one of the electrodes via gateleakage of the transistor. The bias voltage source may include apermanently charged material within the at least one of the electrodes.

In general, in another aspect, an electroacoustic transducer includes ahousing and a diaphragm located proximate to the housing and adapted tomove relative to the housing. The transducer also includes adisplacement sensor, which includes a first electrode adhered to thehousing, a second electrode on a first surface of the diaphragm, locatedproximate to a first region of the first electrode, and a thirdelectrode on the first surface of the diaphragm, located proximate to asecond region of the first electrode. A first capacitance between thefirst electrode and the second electrode and a second capacitancebetween the first electrode and the third electrode each vary similarlywith a displacement of the moving portion relative to the housing. Animpedance buffer is coupled to the second electrode and the thirdelectrode.

The second and third electrode may each include a layer of metal adheredto a top surface of the diaphragm facing away from the first surface ofthe housing, the diaphragm may be attached to the housing by a ringsurrounding a periphery of the diaphragm, and electrical connections tothe second and third electrodes may be made via the ring. The second andthird electrode may each include a layer of metal adhered to a bottomsurface of the diaphragm facing towards the first surface of thehousing, and electrical connections to the second and third electrodesmay be made via the housing where the housing contacts an outerperiphery of the diaphragm. The second and third electrodes may eachinclude a layer of metal adhered to a bottom surface of the diaphragmfacing towards the first surface of the housing, and electricalconnections to the second and third electrodes may be made where a voicecoil is mechanically coupled to the diaphragm.

Advantages include sensing the displacement of the moving structurewithout contacting it, so that the mechanical dynamic performance of thetransducer is not substantially changed by the measurement. Anintegrated sensor may work over a broader frequency range and with lowernoise than a discrete sensor.

Other features and advantages will be apparent from the description andthe claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a cross-sectional plan view of an electroacoustictransducer.

FIGS. 1B and 9 show exploded cross-sectional plan views ofelectroacoustic transducers

FIGS. 2 and 8 show cross-sectional isometric views of electroacoustictransducers.

FIGS. 3, 6A, and 6B show close-up isometric views of an integratedsensor in an electroacoustic transducer.

FIGS. 4A through 4D show close-up cross-sectional plan views of anelectrical connection between components.

FIGS. 5, 7, and 10 show schematic circuit diagrams.

DESCRIPTION

An electromechanical transducer includes an integrated displacementsensor for directly measuring displacement (and its derivatives) of thetransducer. Such a transducer can be advantageous in a feedback controlsystem where perturbations to the transducer are corrected by thecontrol loop. For example, an electroacoustic transducer (10) with anintegrated sensor is shown in FIGS. 1A, 1B, and 2. Transducer 10includes a diaphragm 12, a voice coil 14, a magnetic assembly 16, abasket 18, and an integrated capacitive sensor 100.

When electrical current is applied to the voice coil 14, it interactswith the magnetic field of the magnetic assembly 16 to produce theforces that move the voice coil 14 and diaphragm 12 relative to themagnetic assembly 16 and basket 18 to produce acoustic radiation. Insome examples, the voice coil 14 and at least part of the magneticassembly 16 are reversed, such that the magnetic assembly moves thediaphragm and the voice coil remains stationary relative to the basket.In the particular type of transducer shown, the diaphragm includes adome 20 and a surround or suspension 22. In other types of transducers,a cone may be used to provide additional radiating surface area. In thisexample, the integrated capacitive sensor 100 directly measuresdisplacement of the diaphragm.

Referring again to FIGS. 1A-1B and 2, the magnetic assembly 16 includesa ring magnet 24, a cup 26, and a top plate or coin 28. Other motorstructure geometries may be used, depending on the particularapplication of the transducer. A hole 30 through the magnetic assemblyallows air compressed on the back side of the diaphragm to escape outthe back of the transducer. In some transducers, a limiter 32 seatedatop the top plate physically limits the downward range of motion of thediaphragm, and has holes 34 to allow air to flow between the back sideof the diaphragm and the hole 30 though the magnetic assembly. Thelimiter may be separated from the top plate 28 by a washer 36. A ring 38anchors the outer periphery of the diaphragm 12 to the basket 18. Theparticular physical structures shown here are for illustration only, asthe invention described below may be applicable to any type ofelectroacoustic transducer, however constructed. The physical componentsthat support the active parts of the transducer but do not themselvescontribute to the acoustic function aside from being present in theenvironment, such as the basket 18, washer 36, and limiter 32, arereferred to generically as the “housing.”

The capacitive sensor 100 measures displacement of the diaphragm. Thissensor is shown enlarged in FIG. 3. The capacitive sensor 100 includes alayer of metal 102 on the limiter, another layer of metal 104 on thediaphragm, circuitry 106 and circuit board 108, and various electricalconnections between these components. In transducers lacking the limiter32, a plate may be added or another structure within the transducer maybe used to support the metal layer 102. In the example of FIG. 3, thecircuitry 106 and circuit board 108 are shown inside the limiter, with awire 110 providing a connection to outside circuitry through the hole 30through the magnetic assembly. In other examples, the circuitry and anycircuit board may be located farther from the metal layers, or outsidethe transducer entirely. Electrical connections 112 between the metallayer 102 and the circuit board may be made through a hole 113 throughthe limiter 32. Only one electrical component on the circuit board 108is shown, but additional components may be provided on the circuit boardin other implementations. An example circuit that may be implemented onthe circuit board is described below in connection with FIG. 5.

The layer of metal on the limiter (e.g., layer 102) is referred as theback plate, or electrode, while the layer of metal on the diaphragm(e.g., layer 104) is referred as the front plate (of a two platecapacitor), or electrode. In operation, a bias voltage is applied acrossthe plates, and the circuit elements react to the change in capacitancebetween the plates, which varies with the displacement of the diaphragm,to produce a voltage that is proportional to the change in thedisplacement. Specifically, the charge on the plates is held constant bythe bias voltage, so the changing capacitance changes the voltage acrossthe plates. The change in voltage across the plates is then measured asan indication of displacement. The circuit serves as an impedance bufferto convert the signals produced by the sensor to signals usable byexternal circuitry.

The circuitry may be coupled to the back plate in various ways,depending on the construction of the back plate and the limiter. In oneexample, as shown in FIG. 4A, the entire limiter 32 is made of metal,such as copper. In this example, no separate component is needed toserve as the back plate, and the connection between the limiter 32 andthe circuit board 108 may be made directly, without requiring any wires.For example, such connections could be made by locating contact pads ofthe circuit board 108 at the points where the limiter 32 meets thecircuit board, and solder or conductive paste used to attach the two. Inanother example, as shown in FIG. 4B, the back plate 102 is formed by alayer of metal on top of the limiter 32, and a conductive path throughthe limiter, such as a channel 202 filled with solder or otherconductive material electrically coupled to the back plate 102, providesa connection point that can be connected to the circuit board 108. Inthe example of FIG. 4B, the connection point is spaced from the circuitboard 108 and connected to it by a lead 204. In other examples, theconductive path extends to a part of the limiter already in contact withthe circuit board, so no lead is needed, as in the metallic limiterexample of FIG. 4A. In other examples, a conductive pin may extend fromthe circuit board into the channel 202. In yet another example, as shownin FIG. 4C, the back plate 102 is again provided by a layer of metal ontop of the limiter 32, and the connection between the back plate and thecircuit board 108 is made by a lead wire 206 extending through anopening 208 in the limiter. The back plate 102 in the examples of FIGS.4B and 4C may be formed in any conventional manner of placing a metallayer on another object, including application of foil, metallic paint,electroplating, or vapor deposition, to name a few examples.

In another example, as shown in FIG. 4D, the limiter itself is formedfrom a printed circuit board 210, with the back plate provided by ametal layer 212, and the circuitry 106 and any other electronics mountedto the opposite side of another layer 214 of the combinedlimiter/circuit board 210. The metal layer 212 is shown having acurvature, and there are advantages to this, as explained below. In someexamples, it may be preferred to keep the circuit board 210 flat andcompensate for the benefits provided by the curvature in other ways. Aconductive pathway 216, e.g., a standard via, connects the back platelayer 212 to the other layer 214 in a conventional manner for makingconnections between circuit board layers. Although the connectionbetween the plate 102 and the circuit board 108 is shown in the centerin each example, it may be located at other positions, or multipleconnections may be made. In the example shown, a void beneath thecircuit board 210 accommodates circuit elements such as the circuitry106. If such a void is not present in a given implementation, stand-offsmay be used to create a space for such components between the circuitboard and the structure beneath. Each of these different options may beadvantageous in different situations, dependent on such factors as thesizes or curvatures of the components involved and the manufacturingprocesses used. For example, more discrete components may be used in alarger transducer, where there is more room, and more integratedcomponents may be used in a smaller transducer, though the reverse maybe advantageous in some situations.

In some examples, the entire front surface of the diaphragm, that is,the surface facing the outside of the transducer, is metallized, thoughonly the portion aligned with the metal portion of the back platecontributes significantly to the measured capacitance. Metallized areassurrounding the aligned portions may contribute some small effects. Theelectrical connection to that active portion is made through theremaining metallized region that extends to the outer edge of thediaphragm. The ring 38 (shown in FIG. 2) used to attach the peripheraledge of the diaphragm to the basket is made of copper or anothersuitable conductive material, such that when the entire diaphragm ismetallized, the electrical connection may be made at the ring 38. Thisavoids the need to connect a wire lead to a moving part, a source ofboth distortion and failures in many loudspeakers. In some examples, themetallization may be masked to provide a connection to the outer rimwithout covering the entire surface. The metallized layer of thediaphragm may also be connected by a passage through the diaphragm atthe point where it is joined to the voice coil, or formed on the bottomside of the diaphragm in the first place, allowing the electricalconnection to the metallized layer to be made near the voice coil, thesame point that other connections are made to the moving parts of thedriver. It has been found that a metal layer on the diaphragm as thin asabout 150 Å provides sufficient capacitance to reliably measure thedisplacement of the diaphragm while adding a negligible amount of movingmass. In some examples, the diaphragm material is sufficiently thin thatthe choice of side for metallization does not appreciably affect thecapacitance or the responsiveness of the sensor, and which side of thediaphragm is metallized may be based on manufacturing convenience. Insome examples, the entire diaphragm may be made of a metallic material.

In some examples, the bias voltage is provided by a battery coupledthrough a large bias resistor to the diaphragm plate and back plate.Other sources of bias voltage may include phantom power over the signalconnection, or permanent charges in one or both of the plates, as in anelectric microphone. The external bias voltage, when used, is connectedto one plate of the capacitor through a circuit element, such as afield-effect transistor (FET). This circuitry serves to convert the highoutput impedance of the capacitive sensor into a low output impedancedriver, in some cases with amplification as well, for driving externalcircuitry with reduced susceptibility to noise and interference. Suchcircuitry can be generally referred as an impedance buffer. One exampleof an impedance buffer useful in this situation is a FET connected as acommon-source amplifier. The FET provides a high input impedance so thatit does not load the sensor, and also provides a low output impedancefor communicating the small measured voltages between the plates alonglong signal lines. In addition, the FET works well because its gateleakage provides high impedance required by the bias voltage withoutrequiring a large bias resistor as in a condenser microphone. Varioustypes of field-effect transistors may be used, such as junctionfield-effect transistors (JFET) or metal-oxide field-effect transistors(MOSFET). Other types of amplifiers can also be used for the impedancebuffer, and references to a FET herein is exemplary only—it is notintended to be limiting.

The bias voltage and FET are coupled to the plates and provide an outputsignal as shown in FIG. 5. In this representative circuit 300, one sideof the bias supply 302 is connected to the source terminal of the FET106 at a node 304, and biases the stationary plate through the FET'sleakage impedance. In the example of FIG. 5, this connection alsoincludes a current limiting resistor 322. This optional resistor 322acts as a fuse to prevent the bias supply from damaging the FET if thetwo plates of the sensor are shorted. The other side of the bias supply302 is connected to the front plate on the diaphragm, represented as oneterminal of a variable capacitor 306, at a node 308. In some examples,the diaphragm and the bias supply are coupled through a common ground,rather than through a specific node 308. The polarity of the bias supplymay be reversed from that shown in FIG. 5. In the example that one orboth of the plates is permanently charged, the bias supply 302 isessentially integrated into the plate itself (i.e., into the variablecapacitor 306), and the battery 302 in the circuit diagram of FIG. 5 isreplaced by a virtual high-impedance source. The other terminal of thecapacitor, representing the back plate located on the limiter, iscoupled at node 310 to the gate terminal of the FET 106.

The capacitance of the two parallel plates is found from the well-knownformula:

$\begin{matrix}{{C_{0} = \frac{k\; ɛ\; A}{d_{0}}},} & (1)\end{matrix}$where k is a unitless factor to account for edge effects and istypically ≈1, ∈ is the dielectric constant in air (8.9×10⁻¹⁵ F/mm), A isthe area of the plates in mm², and d₀ is the resting distance betweenthe plates in mm (any suitable system of units may be used). When themoving plate moves by an amount Δd, the capacitance becomes

$\begin{matrix}{C = {\frac{k\; ɛ\; A}{d_{0} + {\Delta\; d}}.}} & (2)\end{matrix}$

From (2), capacitance is non-linearly related to the displacement. Toobtain a measurement that is linearly related to displacement, a biasvoltage e₀ is applied, as in a condenser microphone. A bias voltageapplied across the parallel plates through a high impedance results in aconstant charge Q₀ on the plates, given by:

$\begin{matrix}{Q_{0} = {{C_{0}e_{0}} = {\frac{k\; ɛ\; A}{d_{0}}{e_{0}.}}}} & (3)\end{matrix}$

When the moving plate moves by Δd, the charge Q₀ resulting from the biasvoltage stays the same, but the voltage across the plates changes toe=e₀+Δe, thus

$\begin{matrix}{Q_{0} = {{Ce} = {{\frac{k\; ɛ\; A}{d_{0} + {\Delta\; d}}\left( {e_{0} + {\Delta\; e}} \right)} = {{C_{0}e_{0}} = {\frac{k\; ɛ\; A}{d_{0}}{e_{0}.}}}}}} & (4)\end{matrix}$Solving for the relationship between voltage change and displacementgives:

$\begin{matrix}{{\Delta\; e} = {\frac{e_{0}}{d_{0}}\Delta\;{d.}}} & (5)\end{matrix}$Thus, as the capacitance varies, with a bias voltage applied to theplates, the voltage at the gate of the FET varies linearly with thedisplacement of the diaphragm.

Such a linear output may be more useful in subsequent uses of thedisplacement measurement than an inversely-proportional measurement ofactual capacitance would be. Once displacement is known, its derivativemay be used to provide the velocity of the diaphragm, and that may inturn be derived into acceleration, depending on the signal processingneeds of the device and its users.

The FET amplifies the gate voltage to provide a corresponding voltageacross the source and drain. The source terminal and drain terminal ofthe FET are coupled to the signal outputs 314, 316, as well as toseveral additional components, at nodes 304 and 312. A small capacitor318, on the order of 40-50 pF, across the FET from drain to sourceprovides protection from radio-frequency (RF) noise. A small shuntcapacitor 320, on the order of 0.1 μF, provides protection from noisepickup by the bias supply connections. As mentioned above, the optionalresistor 322 in series with the bias supply, on the order of 100 kΩ,protects the high-voltage bias supply from short-circuiting. Phantompower for the FET is represented by a supply 324 and bias resistor 326across the output lines.

Contrary to what might be expected, the curvature of the plates does notcompromise the measurement or require more-complicated processing. Whilethe curvature does make the measurement less linear, as the fact thatthe two plates remain parallel, that is, the variable distance betweenthe plates is the same at every point, the sensor remains sufficientlylinear for practical purposes. The curvature, by increasing the surfacearea, also helps maximize the total capacitance for a given maximumdisplacement. The curvature of the dome is desirable because itincreases the diaphragm's stiffness and thereby reduces breakup of thediaphragm, which could lead to non-linear performance of both thetransducer and the sensor.

Various types of external sensor may interfere with the dynamicperformance of the transducer, either by accidentally contacting thediaphragm if located too close, or by mass-loading the diaphragm ifattached to it. Because the sensor described herein is integrated intothe transducer, and the added moving mass of metalizing the diaphragm isnegligible, it does not change the dynamic behavior of the transducer inany measurable way, leaving the acoustic performance of the transducerunchanged. An integrated sensor also works over a broad frequency rangewith low noise, as the body of the driver shields the sensor, bothphysically and magnetically, and provides an intimate connection betweenthe sensor and the diaphragm it is measuring. Integrating theelectronics allows the connections between the sensor and the FET to bevery short, reducing interference from outside noise. Integration alsoallows the FET to amplify the signal before it ever leaves thetransducer, providing a large output signal voltage that is lesssusceptible to noise in the signal path. The cost of adding such asensor to a transducer may also be lower than other sensors.

In another embodiment, as shown in FIGS. 6A, 6B, and 7, one of theplates may be split into two electrically isolated parts 402 a and 404 b(FIG. 6A) or 402 b and 404 b (FIG. 6B), with the DC bias applied betweenthose two parts. The other plate 406 a (FIG. 6A) or 406 b (FIG. 6B) isnot connected to the electrical circuit except through capacitance withthe two halves of the split plate. In this embodiment, the capacitancesbetween each of the two split plates 402 a/b and 404 a/b and the solidplate 406 a/b vary together. A differential voltage is created byapplying opposite bias charge on the two parts of the split plate,obviating the need to connect a voltage source or signal connection tothe single plate. In the case of FIG. 6A, this has the advantage of notrequiring connections to the moving diaphragm. In the alternative caseof FIG. 6B, the advantage is in not requiring circuitry inside thetransducer, as the connections from the split plates 402 b and 404 b tothe circuit board 108 may be made externally. Those connections may bemade to the diaphragm in the same manner discussed above, e.g.,metalizing the entire surface of the diaphragm (but for a space betweenthe two plates) and contacting the metalized layer where it is anchoredat the edge, as in FIG. 2.

This split sensor is represented by two variable capacitors 412, 414 inseries. The parts of the capacitor symbols corresponding to plates 402,404, and 406 are also labeled in FIG. 7 for convenience. These twocapacitors 412, 414 each have one-half the capacitance of a single-plateversion with the same total surface area, and their capacitances changewith displacement at the same rate. Each half is biased with half of thesingle-plate bias voltage, but the voltage change from the two halvesadds, so the sensitivity to displacement remains the same as that foundin equation (5) above in the single-plate version. This means that ameasurement circuit 400 that is basically the same as the circuit 300used for the single-plate version, shown in FIG. 5, can be used with thetwo-plate version, as shown in FIG. 7. The only structural difference isthe two variable capacitors 412 and 414 representing the capacitiveplates, though of course the values of specific elements may bedifferent. The two-plate implementation also improves common-moderejection. Because the two split plates 402 and 404 receive the samecommon-mode noise but are opposed to each other in the circuit 400, thatnoise is isolated from the net signal output. While the split plates areshown as identical half-moon shapes in the embodiments of both FIG. 6Aand FIG. 6B, other shapes are possible. For example, they could beconcentric rings, or a ring and a disc. It is preferable that they be ofthe same area, but adjustments for differing areas could be made in thecircuitry. In some examples, a larger number of plates may be used, suchas pie-piece segments, with every other segment having one voltage leveland the other segments the opposite voltage. This may further enhanceresistance to some types of noise, and would be easier to connect toexternal circuitry, in the case of the split plates being on thediaphragm, than concentric plates.

In the split-plate embodiment shown in FIG. 6A, all of theelectrically-connected parts of the sensor are stationary, improving itsmechanical reliability. The sensor components are not subject tomechanical fatigue, there is less moving mass, and the moving parts mayhave better balance and symmetry. Even in the embodiment of FIG. 6B,this advantage is achieved if the connections are made at or beyond thepoint that the diaphragm is anchored to the basket. Avoiding additionalconnections to the moving parts also makes the transducer easier toassemble. This all leads to a more accurate sensor and a more robustpart.

The split-plate embodiment is particularly useful in certain types oftransducers, such as compression drivers, where the diaphragm mayalready be metal, but be difficult to connect to electrically. In acompression driver, the back plate may be formed as a metallized layeron the top surface of the phase plug. A novel type of compression driveris described in U.S. patent application Ser. No. 12/490,463, filed Jun.24, 2009, and incorporated fully here by reference. The phase plug andpart of the surrounding structure and diaphragm from that application isshown combined with the present invention in FIG. 8.

As shown in FIG. 8, the compression driver 500 includes a phase plug 502and a diaphragm 504 (shown raised from its normal position). Thecompression driver's mechanical and electrical structures are notspecifically shown. Part of the phase plug, the bridge 506, ismetallized or made of a separate, metal, part, and provides the twohalves 508, 510 of the back plate. In the example of FIG. 8, the backplates are connected to the biasing and amplifying circuit (not shown)by connections 512 through the body 514 of the motor structure. Theseconnections may be routed, for example, through legs (not shown)supporting the bridge element 506. In other examples, the circuit may beattached to or embedded in the phase plug and may be closer to the backplate, as in the previous examples.

In another embodiment, the fixed plate may be a porous screen located infront of and conformal to the diaphragm, locating the sensor on theoutside of the transducer. In this example, the porosity of the fixedplate avoids significantly changing the acoustic loading on thediaphragm. This construction also avoids having to provide an electricalconnection through the motor structure.

In another embodiment, as shown in FIG. 9, a sensor 1000 is implementedas plates 1002 and 1004 located on the side of the motor structure, suchthat one plate slides past the other, varying the surface area of thecapacitor. In this embodiment, the capacitance varies linearly with thealigned surface area and therefore with the displacement:

$\begin{matrix}{A = {A_{0} - {w \times \Delta\; l}}} & (6) \\{{{\Delta\; C} = {{C_{0} - C} = {{\frac{k\; ɛ\; A_{0}}{d_{0}} - \frac{k\;{ɛ\left( {A_{0} - {w\;\Delta\; l}} \right)}}{d_{0}}} = \frac{k\; ɛ\; w\;\Delta\; l}{d_{0}}}}},} & (7)\end{matrix}$where w and l are the width and length of the plates, A₀=w×l.Displacement is represented by Δl, as the moving plate moves in thedirection of the length, while the gap d₀ remains unchanged. To convertthe capacitance to an output signal representing the displacement, theelectrodes are coupled to an RF bridge circuit 1006 such as that shownin FIG. 10. The circuit board 1008 may be located on the outside of themotor structure, and coupled to the plate(s) by leads 1012 (no lead tothe moving plate is shown). As in the example of FIG. 6, there may betwo plates on the stationary side, removing the need to provide aconnection to the moving plate 1002. In the RF bridge circuit 1006, thecapacitor formed by the moving plates is represented as a variablecapacitor 1010. Fixed capacitor 1012 and resistors 1014 and 1016 causethe voltage at the center of the bridge, amplified by an amplifier 1018,to vary with the capacitance of the plates. A bias supply 1020 providesthe fixed charge to the plates. If two plates are used on the stationaryside, they are represented in the circuit by two capacitors back-to-backas in the example FIG. 7.

Electromechanical transducers include electroacoustic transducers (alsoreferred to as loudspeakers and microphones), linear or rotary electricmotors, and electromechanical sensors. This disclosure is concernedgenerally with transducers that cause or measure small and generallyoscillating movements, where a moving portion of the transducer movesback and forth around a stationary portion. For example, in aloudspeaker, the acoustically-radiating surface, referred to as thediaphragm, and some portion of the motor structure move back and forth,while another portion of the motor structure remains stationary. In someexamples, the moving portion of the motor is a voice coil positionedaround a magnetic structure. In other examples, the voice coil is insidea hollow magnetic structure. In still other examples, the coil isstationary and it is the magnet that moves the diaphragm, or thediaphragm is magnetically responsive and requires no additional movingcomponents. In non-acoustic applications, an electromagnetic linearmotor includes a moving armature and a stationary stator. Either one ofthe armature and stator may include the magnets and the other the coilsor some other mechanism for converting electric energy into motion ofthe armature.

Other implementations are within the scope of the following claims andother claims to which the applicant may be entitled.

What is claimed is:
 1. An electroacoustic transducer comprising: ahousing; a diaphragm located proximate to the housing, adapted to moverelative to the housing; a motor adapted to cause the diaphragm to moverelative to the housing in response to an input current, the motorcomprising a voice coil and a magnet, one of the voice coil or themagnet mechanically coupled to the diaphragm and the other mechanicallycoupled to the housing, the movement of the diaphragm caused by themotor resulting in the transducer outputting acoustic energy; adisplacement sensor comprising: a first plate electrode adhered to thediaphragm, a second plate electrode on a first surface of the housinglocated proximate to a first region of the first plate electrode, and athird plate electrode on the first surface of the housing locatedproximate to a second region of the first plate electrode, wherein afirst capacitance between the first plate electrode and the second plateelectrode and a second capacitance between the first plate electrode andthe third plate electrode each vary similarly with a displacement of thediaphragm relative to the housing caused by action of the motor or byexternal forces acting on the diaphragm; and an impedance buffer coupledto the second plate electrode and the third plate electrode.
 2. Theelectroacoustic transducer of claim 1 wherein the change in distancebetween the first surface of the housing and a surface of the diaphragmresulting from movement of the diaphragm is substantially uniform overthe area of the second and third plate electrodes.
 3. Theelectroacoustic transducer of claim 1 wherein the diaphragm is metal andcomprises the first electrode.
 4. The electroacoustic transducer ofclaim 1 wherein the first surface of the housing is a surface of a blockof conductive material, and the second and third plate electrodescomprise portions of the block that are electrically insulated from eachother.
 5. The electroacoustic transducer of claim 1 wherein the firstsurface of the housing is a surface of a block of non-conductivematerial, and the second and third plate electrodes comprise layers ofmetal adhered to the block of non-conductive material.
 6. Theelectroacoustic transducer of claim 1 wherein the housing comprises abasket.
 7. The electroacoustic transducer of claim 1 wherein the housingcomprises a pole piece.
 8. The electroacoustic transducer of claim 1wherein the layer of metal of the first electrode comprises a coating ona non-conductive substrate.
 9. An electroacoustic transducer comprising:a housing; a diaphragm located proximate to the housing and adapted tomove relative to the housing; a motor adapted to cause the diaphragm tomove relative to the housing in response to an input current, the motorcomprising a voice coil and a magnet, one of the voice coil or themagnet mechanically coupled to the diaphragm and the other mechanicallycoupled to the housing; the movement of the diaphragm caused by themotor resulting in the transducer outputting acoustic energy; adisplacement sensor comprising: a first plate electrode adhered to thehousing, a second plate electrode on a first surface of the diaphragmlocated proximate to a first region of the first plate electrode, and athird plate electrode on the first surface of the diaphragm locatedproximate to a second region of the first plate electrode, wherein afirst capacitance between the first plate electrode and the second plateelectrode and a second capacitance between the first plate electrode andthe third plate electrode each vary similarly with a displacement of thediaphragm relative to the housing caused by action of the motor or byexternal forces acting on the diaphragm; and an impedance buffer coupledto the second plate electrode and the third plate electrode atnon-moving points at a periphery of the diaphragm.
 10. Theelectroacoustic transducer of claim 9 wherein: the first surface of thediaphragm is a top surface of the diaphragm; the second and third plateelectrode each comprise a layer of metal adhered to the top surface ofthe diaphragm facing away from a surface of the housing; the diaphragmis attached to the housing by a ring surrounding a periphery of thediaphragm; and electrical connections to the second and third plateelectrodes are made at the periphery of the diaphragm.
 11. Theelectroacoustic transducer of claim 9 wherein: the first surface of thediaphragm is a bottom surface of the diaphragm; the second and thirdplate electrode each comprise a layer of metal adhered to the bottomsurface of the diaphragm facing towards a surface of the housing; andelectrical connections to the second and third plate electrodes are madevia the housing where the housing contacts an outer periphery of thediaphragm.